1. Field of the Invention
The present invention generally concerns operation of thermal infrared (IR) radiation sensors.
The present invention particularly concerns operating arrayed thermal infrared (IR) radiation sensors other than by use of a light chopper to periodically modulate, or interrupt, incoming light radiation so as to cause the sensors to produce an alternating current electrical signal, as is conventional. Instead, the present invention concerns operating arrayed thermal infrared (IR) radiation sensor elements by (i) continuously illuminating the arrayed sensor elements with radiation from a scene, each arrayed sensor element coming to an equilibrium temperature corresponding to an associated pixel of the scene, and (ii) occasionally or periodically applying a transient heat pulse to momentarily displace each sensor element from equilibrium.
2. Description of the Prior Art
2.1 General Prior Art Radiation Sensors
Classical uses of thermal radiation sensors include (i) military applications in surveillance, for example in infrared night vision and/or intruder detection; (ii) civilian applications in non-contact temperature measurement, for example in remote fire alarms, process control, and quality analysis; and (iii) medical applications, for example in burn healing analysis.
The two main methods of sensing infrared (IR) radiation are either (i) to use the incident photon flux to excite carriers within a material (i.e., photon detection), or (ii) to use a substance with a strongly temperature dependent property (i.e., thermal detection).
Generally, photon detectors tend to exhibit a higher speed of response than do thermal detectors. Photon detectors also exhibit good quantum efficiency up to their cut-off wavelength, which is determined by the energy of the corresponding transition. The transition sets the level of cooling required: the lower the energy band gap of the transition that is responsible for photon sensing, the longer the radiation wavelengths that can be detected. However, the temperature of the photon sensor must also be kept low so that the noise corresponding to the temperature generated carriers does not dominate the signal corresponding to the photon generated carriers. Typically, photon sensors able to detect incident radiation of 14 .mu.m wavelength need to be cooled to liquid nitrogen temperature (77.degree. K.) or lower.
Thermal detectors tend to he slower that photon sensors because heating and cooling of a macroscopic sample is a relatively slow process. However, these thermal effects do not depend on the nature of the incident radiation that is absorbed by or coupled to the thermal sensing element. In many instances coatings are used to enhance absorption in a portion of the spectrum that is of particular interest. Some thermal sensors can operate over a wide temperature range (though cooling generally improves their performance), while, depending on the properties of the material, other thermal sensors must be maintained within a temperature window.
The ability to operate at or close to room temperature is an important characteristic of low-cost and/or low-weight thermal sensors that do not take advantage of cryogenic cooling. Two popular kinds of thermal sensors are 1) the bolometer and 2) the pyroelectric sensor.
The bolometer can be considered to be a resistor endowed with a resistance that varies with temperature. This variable resistance can be measured with appropriate circuits corresponding to the magnitude of the resistance and the change in resistance. Bolometers are often used in staring arrays (i.e., without any such chopper of the incident radiation as will be discussed).
Using a ferroelectric material can lead to two readout mechanisms: 1) the pyroelectric effect, and 2) the dielectric effect (also termed "induced pyroelectricity"). In both cases the sensor can be apprehended to serve as a capacitor.
The dielectric effect relies on the fact that the dielectric permittivity .epsilon. of the sensor's material varies with temperature. Accordingly, once an applied bias has charged the sensing capacitor, a temperature change results in a signal voltage across the element. This mode of operation is often called a "dielectric bolometer".
The pyroelectric effect results in the release of charges across the sensing capacitor. When a pyroelectric element of area A and pyroelectric coefficient p undergoes a change of temperature .DELTA.T, an amount of charges p.multidot.A.multidot..DELTA.T is generated. (Conventionally, under 0 applied field and below the Curie temperature T.sub.c, the pyroelectric coefficient p=dP.sub.s /Dt, where P.sub.s is the spontaneous polarization of the ferroelectric material). The charges can be sampled directly by an electrometer type mechanism, as the voltage across the element, or as the current resulting from connecting the electrodes to an external circuit.
In an infrared radiation sensing system, the radiation coming from a scene or an object are imaged onto thermal sensors formed as an array of pixels. Each sensor within the sensor array senses radiation of a magnitude related to the amount of radiation emanating from the corresponding portion of the scene.
When the radiation sensors employ the pyroelectric effect, surface charge recombination and readout leakage tend to cancel out the effect over time. Accordingly, an alternating, AC, sampling mode is necessary. Other types of sensors (e.g., bolometer) have also been used in AC mode.
The AC mode is realized by chopping the incident radiation, meaning that this radiation is from time to time attenuated or, most commonly, interrupted. The input radiation signal to the array is thus alternating, or AC. The thermal response of the radiation sensors within these conventional, chopped, systems is typically linear with temperature. All the pixel sensors thus commonly oscillate approximately around the same equilibrium temperature that is determined by (i) the incident radiation, and (ii) thermal coupling of the sensors with the substrate. The AC electrical signals produced by the sensors in response to the chopping are usually fed into differential amplification circuitry, which helps to reduce the amount of noise in the system compared to staring arrays that operate in DC mode.
The problem with so chopping the incident radiation is that some radiation is lost and never reaches the sensors, reducing the sensitivity of the system. The proposed invention addresses the problem associated with using thermal infrared sensors that are operated in conjunction with a light chopper that modulates the incoming radiations to produce an alternating signal.
It will be seen that, when the present invention is used, a cumbersome chopper is no longer necessary, yet the corresponding sensor system will retain all the advantages of AC sampling, and some other characteristics will be improved as well.
2.2 Specific Prior Art Radiation Sensors
A system concerned with like objects of thermal radiation sensing as is the present system is shown and described in U.S. Pat. No. 5,486,698 to Hanson, et. al. issued Jan. 23, 1996 for a THERMAL IMAGING SYSTEM WITH INTEGRATED THERMAL CHOPPER and assigned to Texas Instruments, Incorporated (Dallas, Tex.). The Hansen, et al. patent concerns a thermal imaging system that contains a focal plane array including a plurality of thermal sensors mounted on a substrate. The focal plane array generates both a reference signal which represents the temperature of the substrate and a biased signal corresponding to the total radiance emitted by a scene. Electronics process the reference signal and the biased signal to obtain an unbiased signal representing radiance differences emitted by objects in the scene. A thermoelectric cooler/heater may be provided to optimally adjust the temperature of the substrate to improve overall image quality. Each thermal sensor contains an electrode that electrically couples the thermal sensor to the substrate and also allows the thermal sensor to deflect, contact, and thermally shunt with the substrate.
More remote to the present invention, U.S. Pat. No. 5,302,830 to Shivanandan--issued Apr. 12, 1994 for a METHOD FOR MEASURING THERMAL DIFFERENCES IN INFRARED EMISSIONS FROM MICRO DEVICES and assigned to General Research Corporation (Vienna, Va.)--describes a method for measuring thermal differences in infrared emissions from semiconductors. In the method an image sensor includes an array detector having a plurality of detector elements which produce signals corresponding to semiconductor radiation emission focused thereupon by an optical lens system. At least one bandpass filter is utilized to substantially filter that portion of the semiconductor radiation emission having wavelengths greater than 5 micrometers. The detector element signals are processed to identify performance degrading phenomena occurring in the semiconductor device.
U.S. Pat. No. 5,546,041 to Szajda--issued Aug. 13, 1996 for a FEEDBACK SENSOR CIRCUIT and assigned to the Massachusetts Institute of Technology (Cambridee, Mass.)--concerns a circuit for sensing a parameter such as temperature. The circuit has a single current source and a mismatched diifferential pair for providing two different currents at a desired current ratio. Each current is provided to a current control element and a sensing element, each of which may be diodes. An operational amplifier closes the loop and feeds back a signal from the input of the current control elements to the differential pair to hold the desired current ratio constant. Chopper circuitry is employed with the amplifier to reduce the offset and low frequency noise contribution of the amplifier.